The present invention relates to a method of producing an MIS transistor having a gate electrode of the same conductivity type as that of the source and drain regions of the transistor. Hereinafter, such gate electrode is referred to as a "matched conductivity type" gate electrode.
Conventionally, when producing an MIS transistor having a gate electrode of matched conductivity type, an ion implantation method or a predeposition method is utilized to dope the same conductivity type of impurity into the gate electrode and the source and drain regions.
However, the conventional doping methods have various drawbacks. With regard to the ion implantation method, the impurity particles are implanted to a certain depth due to the kinetic energy of the accelerated impurity ion particles. Therefore, when activation of the ion particles is carried out by thermal treatment after the ion implantation, the impurity particles are diffused from the gate electrode to a gate insulating film and further to a channel region.
With regard to the predeposition method, since the impurity is diffused through an oxide film, the impurity density cannot be controlled well. Further, since this technique involves relatively high temperature processing at about 900.degree. C., impurity particles may be diffused from the gate electrode into the channel region similarly as in case of the ion implantation.
As noted above, when the conventional methods such as ion implantation and predeposition are utilized to effect the impurity doping to a gate electrode of a matched conductivity type MIS transistor, the impurity particles having one conductivity type opposite to that of the channel region are inadvertently diffused from the gate electrode into the channel region disposed under the gate electrode, thereby causing drawbacks such as shift of threshold voltage of the transistor, and reduction of carrier mobility due to excess density of impurity atoms in the channel region. Such drawbacks are most serious in the P channel MIS transistor having the matched conductivity type gate electrode which contains P type impurity of boron (B), which has a relatively great diffusion coefficient.